This sounds like a nature vs. nurture discussion.

Nature + Nurture = Who the person grows up to be.

Genes (nature) are the functions but life experiences, environment, etc. (nurture) are the parameters.

GeneX(Twin1, ExperienceA) { return x; }

GeneX(Twin2, ExperienceB) { return y; }

The genes are like restricted scales and within each hold all possible outcomes for an individual. Some of these genes have a greater influence on the end result of an individual than others. Different life experiences, environments, etc. result in shaping each twin differently within each of their genes - since even twins cannot live identical lives and experience the same exact experiences.